The hypothesis is that cardiac sarcoplasmic reticulum (SR) is subspecialized. The goal of the proposed research is to document this with an in vitro biochemical approach. A heterogeneous cardiac SR preparation will be centrifuged into subpopulations of ryanodine dine-sensitive (RS) and ryanodine-insensitive (RI) SR vesicles, and the different functional, compositional, and morphological properties of the two subpopulations of SR vesicles determined. Functional studies will address Ca2 plus metabolism. Preliminary data indicate that RI vesicles have a high Ca2 plus pump density and a high capacity for Ca2 plus uptake. Ca2 plus uptake by RS vesicles is selectively stimulated by the drug, ryanodine, which may block distinct Ca2 plus efflux channels present only in these vesicles. Compositional studies will characterize unique proteins in either subpopulation of vesicles which are responsible for their different Ca2 plus handling properties, and morphological studies will determine the subcellular localization of the two types o SR in intact meocardium. Although others have isolated functionally different subpopulations of SR from skeletal muscle, this study is the first to attempt this with cardiac preparations. This study will further clarify the role of SR in regulating excitation-contraction coupling in mammalian myocardium.